feat(scripted_tool): shared context across tool callbacks

[chaliy]

Summary

Add a mechanism for ScriptedTool callbacks to share state (HTTP clients, auth tokens, database connections) without each callback capturing its own copy.

Motivation

Cloudflare's Code Mode MCP injects an authenticated cloudflare.request() function. The LLM never sees raw API keys — auth is handled by the runtime.

ScriptedTool already supports this via closure captures and .env(), but there's no standardized pattern. Each callback must independently capture shared resources, leading to boilerplate and Arc-juggling.

Design options (need discussion)

Option A: TypeMap context

let client = Arc::new(build_authenticated_client());

ScriptedTool::builder("api")
    .context(client.clone())   // registers Arc<ReqwestClient> by type
    .tool(
        ToolDef::new("get_user", "..."),
        |args: &ToolArgs, ctx: &Context| {
            let client = ctx.get::<ReqwestClient>()?;
            // ...
        },
    )
    .build()

Pros: Type-safe, no string keys, familiar pattern (actix-web, axum).
Cons: Changes ToolCallback signature (breaking change to public API).

Option B: Named context with Any

ScriptedTool::builder("api")
    .context("http_client", Arc::new(client) as Arc<dyn Any + Send + Sync>)
    .tool(...)
    .build()

Pros: Named, flexible.
Cons: Runtime type errors, string keys.

Option C: Keep closures, document the pattern

Don't change the API. Document the recommended pattern:

let client = Arc::new(build_authenticated_client());

let c = client.clone();
builder.tool(ToolDef::new("get_user", "..."), move |args| {
    let resp = c.get(&format!("/users/{}", args.param_i64("id").unwrap())).send()?;
    Ok(resp.text()?)
});

Pros: No API change, zero overhead.
Cons: Boilerplate with many tools, each needing let c = client.clone();.

Recommendation

Start with Option C (document the pattern) and revisit Option A if users hit pain points. The ToolCallback signature is public API — changing it requires a major version bump per spec 009.

Acceptance criteria

• Decide on approach (A, B, or C)
• If A or B: implement Context type, update ToolCallback signature, update spec 009 (breaking change)
• If C: add documented example showing shared auth client pattern
• Tests demonstrating shared state across multiple tool callbacks
• Update spec 014

Effort

Small (Option C) to Medium (Option A/B).